1. Field of the Invention
The present invention relates to electrically controlled optical devices with a high operating speed, such as a light intensity modulator, a light phase modulator, or an optical switch, and also to a method of producing such electrically controlled optical devices.
2. Description of the Prior Art
For a better understanding of conventional electrically controlled optical devices, one example of a conventional Mach-Zehnder light intensity (optical) modulator is shown in FIGS. 1A and 1B, which are a plan view thereof and a cross-sectional view taken along the line A--A', respectively. In this example, Mach-Zehnder optical waveguides 2 are formed by Ti thermal diffusion in a Z-cut substrate 1 of LiNbO.sub.3 having electro-optic effects. A buffer layer 3 of SiO.sub.2 having a thickness D (usually, D is 2000 to 3000 .ANG.) is formed on the substrate 1, and a coplanar waveguide (CPW) composed of a center electrode 4 and earth or ground electrodes 5 is formed on the buffer layer 3. Each termination resistor 6 is connected between the CPW electrodes 4 and 5. A feeder 7 is connected to the electrodes 4 and 5 to supply a modulating microwave signal thereto.
The electrode 4 and the electrodes 5 are generally called "CPW electrodes", and are constituted as traveling wave electrodes. With respect to the dimensions of these electrodes, the thickness of the electrodes 4 and 5 is usually 1 to 3 .mu.m, the width 2W of the center electrode 4 is 35 .mu.m, and the gaps 2G between the center electrode 4 and the earth electrodes 5 are 6 .mu.m. Since the characteristic impedance of the CPW electrodes is 21 .OMEGA., 42 .OMEGA. is selected as the value of the termination resistors 6.
Generally, in an optical modulator, when driving electric power is supplied from the modulating microwave signal feeder 7, an electric field is applied between the center electrode 4 and the earth electrodes 5. Since the LiNbO.sub.3 substrate 1 has electro-optic effects, a change in the refractive index occurs due to this electric field. As a result, the light beams propagated respectively through the two optical waveguides 2 are shifted in phase from each other. When this phase shift reaches the level of .pi., the light radiates in to the substrate at a light-mixing portion of the Mach-Zehnder optical waveguide, so that an OFF condition develops. In this optical modulator, since the CPW electrodes 4 and 5 are constituted as traveling wave electrodes, such a limitation as is encountered in an electrical circuit is ideally not imposed on the bandwidth. Furthermore, insofar as the modulating microwave signal wave propagated through the CPW electrodes 4 and 5 agrees in propagating velocity with the light propagated through the optical waveguide 2, the modulation bandwidth is not limited, and therefore the optical modulator can operate at high speed.
Actually, however, there is a difference between the velocity of the microwave and the velocity of the light, and th modulation bandwidth is limited by it. A 3 dB modulation bandwidth .DELTA.f resulting from this velocity mismatch is expressed by the following formula (Reference literature: Journal of Institute of Electronics and Communication Engineers of Japan, (C), J64-C, 4, p. 264-271, 1981): EQU .DELTA.f=1.9 c/(.pi.L .vertline.n.sub.m -n.sub.o .vertline.) (1)
where n.sub.m represents the microwave effective index of the substrate 1 with respect to the signal wave, n.sub.o represents the effective index encountered by the light, L represents the length of the portion of the CPW electrode 4, 5 interacting with the optical waveguide 2, and c represents the light velocity.
Here, the modulation bandwidth .DELTA.f represents the 3 dB modulation bandwidth in optical power. In the above reference literature, the 3 dB modulation bandwidth is indicated in terms of the electric level when the above optical power is converted by a light-receiving device. Therefore, the factor "1.9" in the formula (1) is obtained through conversion of a factor "1.4" of the above reference literature.
The relation of the above microwave effective index n.sub.m with an effective dielectric constant .epsilon..sub.eff (effective relative permittivity) of the substrate 1 is expressed in the following: ##EQU1##
In a substrate material having electro-optic effects, the microwave effective index n.sub.m with respect to a signal wave is usually greater than the effective index n.sub.o with respect to light. The effective dielectric constant .epsilon..sub.eff of the substrate 1 is determined mainly by the dielectric constant .epsilon..sub.r2 and the thickness of the substrate material, the gap 2G between the CPW electrodes 4 and 5, the operating frequency, etc. To permit the substrate to be easily handled when producing it, the thickness of the substrate is usually 0.5 mm to several millimeters. Since the thickness of the substrate is usually sufficiently greater than the electrode gap 2G, the following formula is established: EQU .epsilon..sub.eff .apprxeq.(.epsilon..sub.r2 +1)/2 (3)
The dielectric constant of the LiNbO.sub.3 substrate is nearly equal to 35 (i.e., .epsilon..sub.r2 .apprxeq.35), and from the formulas (2) and (3), the microwave effective index n.sub.m is nearly equal to 4.2 (i.e., n.sub.m .apprxeq.4.2). Since the effective index n.sub.o with respect to light is 2.1 (i.e., n.sub.o =2.1), the microwave effective index n.sub.m is about twice the effective index n.sub.o.
Therefore, when operation of the device is to be effected at 9 GHz, around 10 mm is selected, from the formula (1), as the length L of the portion of the CPW electrodes 4 and 5 interacting with the optical waveguide.
In order that the conventional optical modulator shown in FIGS. 1A and 1B can operate at high speed, it will be appreciated from the formula (1) that the length L of the CPW electrodes 4 and 5 should be shortened in accordance with the operating frequency. However, when the electrode length L is shortened, the driving voltage of the optical modulator becomes higher, which results in the disadvantage that the modulation efficiency is lowered.
FIGS. 2A and 2B show an optical modulator disclosed by some of the present inventors and others in an earlier Japanese Patent Application, Laying-Open No. 48,021/89. In this optical modulator, a shield conductor 9 is provided on electrodes 4 and 5 through an overlaid layer 8. The overlaid layer 8 is made of polyimide or air. By reducing the thickness H of the overlaid layer 8 (that is, bringing the shield conductor 9 closer to the electrodes 4 and 5), the microwave effective index n.sub.m can be decreased, thereby decreasing the velocity mismatch between the microwave and the light.
However, in this optical modulator of the above earlier application, the effect of decreasing the microwave effective index n.sub.m by the buffer layer 3 is not efficiently utilized. Therefore, if it is intended to sufficiently match the velocity of the microwave to that of the light, the characteristic impedance of the CPW electrodes 4 and 5 decreases too much, as compared with the case where the shield conductor 9 is not used. As a result, the CPW electrodes 4 and 5 can not match an external microwave circuit. Moreover, if in view of this impedance decrease due to the shield conductor 9, the gap width 2G is increased (that is, the characteristic impedance is set to a high value), the intensity of the electric field acting on the optical waveguide 2 is weakened, so that the driving voltage is increased. Further, since the shield conductor 9 is disposed close to the CPW electrodes 4 and 5, the microwave conductor loss becomes large as will be later described.
FIGS. 3A and 3B show an optical modulator disclosed by some of the present inventors and others in an earlier Japanese Patent Application, No. 201491/88. In this optical modulator, the width 2W of the center electrode 4 is approximately equal to that of the optical waveguide 2, and the buffer layer 3 of SiO.sub.2 has an increased thickness D, thereby improving the characteristics.
The characteristics of this earlier example are shown in FIG. 4. FIG. 4 shows the results of analysis of the relation of the microwave effective index n.sub.m and the characteristic impedance Z.sub.o with the thickness D of the buffer layer 3 when the width 2W of the central conductor portion 4 of the CPW is 8 .mu.m, and the gap width 2G is 15 .mu.m. In this analysis, the evaluation is made using the spectral domain method (Kawano: "Hybrid-mode analysis of a broadside-coupled microstrip line", IEE Proc. Pt. H, vol.131, pp.21-24, 1984), which can strictly deal with a dielectric multi-layer structure.
As can be seen from FIG. 4, with an increase of the thickness of the buffer layer 3, the microwave effective index n.sub.m decreases, and hence .vertline.n.sub.m -n.sub.o .vertline. decreases. As a result, the product of the 3 dB modulation band and interaction length (.DELTA.f.multidot.L) can be increased, as shown in FIG. 5.
In FIG. 5, the ordinate axis represents the product (.DELTA.f.multidot.L) of the 3 dB modulation band .DELTA.f and the length L of the CPW electrode. Hereinafter, .DELTA.f.multidot.L or .DELTA.f will be often referred to as the "(3 dB) modulation band".
When the voltage for switching the light output from an ON condition to an OFF condition (that is, the half-wave voltage V.pi.) is small, a higher efficiency is achieved. FIG. 6 shows the relation between the thickness D of the buffer layer 3 and the product (V.pi..multidot.L) of the half-wave voltage V.pi. and the interacting length L.
As indicated in the formula (1), the modulation band .DELTA.f is in inverse proportion to the mismatch between the effective index n.sub.m of the microwave and the effective index n.sub.o of the light. Therefore, as shown in FIG. 5, as the thickness D of the buffer layer becomes large, and as the mismatch between the effective index n.sub.m of the microwave and the effective index n.sub.o becomes small, the rate of increase of the modulation band becomes high.
On the other hand, as shown in FIG. 6, V.pi..multidot.L increases substantially linearly with an increase of the thickness D of the buffer layer 3. The ratio of .DELTA.f.multidot.L to V.pi..multidot.L (.DELTA.f/V.pi.) represents a modulation index, and it is desirable that this value be high. As shown in FIG. 7, .DELTA.f/V.pi. tends to increase with an increase of the thickness D of the buffer layer 3, but a marked improvement in optical modulation efficiency can not be expected.
Also, as shown in FIG. 4, when the thickness of the buffer layer 3 is increased, the value of the characteristic impedance Z.sub.o tends to increase, so that it is difficult to achieve matching with an external circuit. Therefore, in view of the impedance matching with the external circuit, if Z.sub.o is determined to be about 50.+-.10 .OMEGA., then it is necessary that the thickness D of the buffer layer 3 be set to not more than about 1 .mu.m.
Accordingly, as is clear from FIG. 7, .DELTA.f/V.pi. is not more than 1.7. Also, when the length L is increased, the microwave conductor loss and the value of the dc resistance are increased, which makes it difficult to obtain good modulation characteristics.
FIG. 8 shows an optical modulator invented by Mitomi et al. and disclosed in Japanese Patent Application Laying-Open No. 91,111/89. FIG. 9 shows an optical modulator proposed in the Second Optoelectronics Conference (OEC '88) Post-Deadline Papers Technical Digest, PD-1 (Oct. 1988, Tokyo).
In FIG. 8, the thickness of traveling wave electrodes 10 and 11 (here, asymmetrical coplanar strips are used as one example) is increased in an attempt to decrease the microwave effective index n.sub.m. With this construction, the characteristic impedance Z.sub.o is also decreased together with the microwave effective index n.sub.m.
FIG. 9 shows a structure which improves such characteristics. In this case, the thickness D of the buffer layer 3 is increased to about 1 .mu.m so as to increase the characteristic impedance Z.sub.o, thereby compensating for the reduction of Z.sub.o caused when the thickness of electrodes 10 and 11 is increased to 10 .mu.m. Further, as described above for the prior art of FIG. 3 with reference to the formula (1) and FIG. 7, the increase of the thickness of the buffer layer 3 also improves the value of .DELTA.f/V.pi.. However, in order to achieve velocity matching between the light and the microwave, it is necessary to set the thickness of the electrodes to not less than 10 .mu.m, and manufacturing difficulties, such as a short circuit between the electrodes 11, are inevitably encountered. Therefore, with this technique, it is difficult to carry out light modulation over a very wide band.
As described above, with the conventional constructions, the matching of the characteristic impedance can not be sufficiently achieved, or the microwave conductor loss can not be kept to a low level, or the half-wave voltage can not be decreased, or the velocity of the microwave can not match the velocity of the light wave. Therefore, it has been very difficult to achieve both a wide band characteristic and a low driving voltage characteristic.